Filter devices of this type are readily available on the market in various versions. To a large extent, such filter devices are used for filtration of working fluids such as hydraulic fluids, fuels, lubricants, and the like. In fluid systems, specifically hydraulic systems, in which filter devices are used, the operational reliability of the system depends largely on the reliable operation of the filter devices located in the system. In other words, a failure or malfunction of the filter device can lead to failure or serious damage of the pertinent system and thus can cause significant economic loss.
A filter device and its parts as well as a method for operating the filter device are known from DE 10 2006 039 826 A1. The filter device has a filter housing part which encloses the filter element and which can be detachably connected to another filter housing part. A valve apparatus in the interconnected state of the housing parts clears a fluid path and, in the separated state of the housing parts from one another, at least partially blocks it. The valve apparatus preferably has a valve element which can be actuated via an actuating part of at least one of the two filter housing parts and/or of the filter element. Especially in the connected state of the housing parts, it opens the valve element and, in its state separated from one another, at least partially closes it. In this case, the first filter housing part forms a filter bowl for receiving the filter element. The second filter housing part forms the filter bowl with the fluid connections, especially in the form of a supply and drain. When the filter element is missing and the valve apparatus is closed, a control unit of the hydraulic circuit delivers a fault message and/or shuts off the hydraulic circuit.
DE 92 15 351 U discloses a filter device for filtering of fluids with a filter element and with a back pressure and an overload valve located in the bypass to the filter element and the back pressure valve. The filter device has a first connection site for supply of the unfiltered fluid from a load site which is supplied as part of a fluid circuit by a fluid pump and a second connection site between the filter element and the back pressure valve for discharging the filter fluid by a differential pressure-controlled valve located in the bypass to the filter element. Depending on the differential pressure on the filter element, a connection between the second connection site and a third connection site can be established which can be connected to the fluid circuit between the fluid pump and the load site. In this case, a switching valve can be provided which, in the absence of the filter element, establishes this connection between the second connection site and the third connection site. Furthermore, a movable element receiver for the filter element can be provided which, at a definable threshold value of the differential pressure on the filter element that is smaller than the threshold value necessary to open the differential pressure-controlled valve, travels against a stop against the force of the energy storage mechanism of the valve piston.